1. Field of the Invention
The present invention relates to an unlubricated sliding member which is used under a condition in which supply of a lubricating oil or the like is difficult or impossible.
2. Description of the Prior Art
A sliding member is generally composed of a first sliding body having a first sliding part and a second sliding body having a second sliding part which comes into sliding contact with the first sliding part.
In such a sliding member, an attempt of producing the first sliding part and the second sliding part from a ceramic material has been made. However, since ceramic has poor toughness , the first and second sliding parts which are made of a ceramic material are susceptible to surface damage during sliding. As a result, the abrasive wear progresses and the sliding member is apt to have a comparatively short durability.
It is known that the surface damage produced on a ceramic sliding member is caused by a tensile stress which is produced in the vicinity of the sliding surface and is called "tensile spike".
The tensile stress which is called tensile spike depends on the friction coefficient of the sliding surface and if the friction coefficient is reduced, the tensile stress is reduced.
In order to reduce the friction coefficient, a lubricating oil is applied to the sliding surface or a solid lubricant is contained in the sliding surface However, a lubricating oil is apt to deteriorate, in particular, when it is used in the place which receives radiation or at high temperature.
As a sliding member which has a solid lubricant in the sliding surface, there are known a sliding member with molybdenum disulfide or fluorocarbon resin immersed into a porous ceramic sintered body, which is disclosed in Japanese Patent Laid-Open No. 118080/1982, and a sliding member having a sintered body consisting of a mixed powder of a ceramic powder and molybdenum disulfide, which is disclosed in Japanese Patent Laid-Open Nos. 188474/1982 and 114028/1982. However, even if a solid lubricant is contained, the friction coefficient is about 0.13, the value being not always satisfactory.
As an example of a sliding member which does not use a lubricating oil or a solid lubricant, there are known sliding members consisting of a zirconia or silicon nitride substrate with Ti, Ni or Co deposited thereon and further subjected to Ar.sup.+ implantation, and a Ti-C-Ni-Mo pin. Such sliding members and their friction coefficients are reported in Friction and Wear of Ion Beam Modified Ceramics; J. Matter, Sci., 22 PP 2069 to 2087 (1987). In this report, a friction coefficient between the partially stabilized zirconia surface with a Co thin film formed thereon and a TiC cement bonded with Ni and Mo (TiC(Ni, Mo)), and a friction coefficient between the partially stabilized zirconia surface with a Ti and Ni thin film formed thereon and TiC(Ni, Mo) are shown. These friction coefficients are 0.08 and 0.09, respectively, at a temperature of 800.degree. C. in the atmosphere of an exhaust gas of a diesel engine. These values cannot be said to be satisfactory values for practical use, and the condition for use of such a sliding member is limited. In addition, the friction coefficients at room temperature are more than 0.2.